Abstract Expert panels, researchers and the NIH have recommended examining multiple familial influences and interpersonal relationships within the home environment in order to identify modifiable risk and protective factors for child and adolescent obesity. The main aim of the proposed study, all in the Family, is to examine the relationships between multiple family members' weight and weight-related behaviors and adolescents' body mass index (BMI) z-score, overweight status, dietary intake and physical activity. This study will address unanswered methodological questions relevant to investigating and intervening on factors that may influence adolescent obesity within the home environment. (1) First, it is unknown whether parents treat siblings within the home similarly (or differently) n regards to weight and weight-related behaviors. Knowledge about differential treatment of siblings by parents will inform whether obesity prevention interventions need to be tailored to different children within the home. (2) Second, it is important to determine who to include in adolescent obesity interventions; should both parents be included and should siblings be involved? Knowing which family members' weight and weight-related behaviors are more highly associated with adolescents' weight, dietary intake and physical activity may give insight into which family member(s) should be included in interventions to prevent adolescent obesity. (3) Third, a critical unanswered question in regards to familial influences on adolescent obesity is whether it is important to include multiple family members' reports of the home environment. For example, do the father or siblings provide unique information about the home environment above and beyond reports by the target child or the mother? The proposed study will answer these important questions by conducting a secondary data analysis using two NIH R01 studies that are multi-level, population-based studies. The proposed study, All in the Family, will use a subset of data from Project F-EAT (Families and Eating and Activity in Teens) and EAT 2010 (Eating and Activity in Teens), which link parent/caregiver (custodial, non-custodial, step-parent) other caregivers (grandmother, aunt), adolescent child, and sibling data. The overall dataset includes 3,709 ethnically/racially and socio-demographically diverse parents (62% female), 2,793 adolescents (53% female), and a subset of 204 adolescent siblings (57% female).The inclusion of sibling data was not originally planned, nor were analyses regarding siblings addressed in the original study aims for EAT 2010 or F-EAT. Thus, the proposed study will uniquely add to the results from EAT 2010 and F-EAT by analyzing data from siblings to identify dyadic and triadic relationships between family members and examining the association between these relationships with adolescent weight and weight-related outcomes. This rich dataset will allow for the evaluation of a more complex interactive model of the familial dynamics contributing to adolescent weight, dietary intake and physical activity that will directly inform te development of more effective family-based adolescent obesity prevention interventions.